For many years, electrolytic processes have been used to produce copper, zinc, nickel, manganese, cobalt and other metals in high purity. These processes fall into two broad categories, electrowinning and electrorefining. In electrowinning, an ore is leached with an acid to obtain the metal sought in solution. The solution is partially purified by chemcial steps and then introduced into electrolytic cells where the metal is removed in high purity by deposition onto a cathode. Zinc is commonly produced by electrowinning and zinc cells employ lead-silver alloy anodes and aluminum cathodes.
In electrorefining, an ore is smelted to produce a metal as a solid at an intermediate level of purity. The solid metal is then used as the anode in an electrolytic cell and is gradually dissolved under a potential to deposit the metal in high purity on a cathode. Copper is commonly produced in this way, and the molten output of the smelter is cast into anodes which contain 95% Cu. In copper cells these anodes are dissolved to produce pure copper on cathodes which are initially thin sheets of pure copper. During the electrochemical step, impurities in the copper anode do not dissolve in the electrolyte but fall to the bottom of the cell and are recovered. These impurities can include gold, silver and metals of the platinum group and in some ores recovery and subsequent purification of impurities has substantial economic significance. In copper refineries, there are auxiliary banks of electrolytic cells used to produce thin copper starter sheets and to recover metal values from various process streams. These cells use lead-calcium alloy anodes.
About 1975, the mining industry undertook substantial efforts to develop alternative processes to eliminate the smelting step. There were two major reasons for this. In smelting, very large amounts of ore are heated to high temperatures, so fuel consumption is large and cost of fuels was increasing rapidly. Secondly, smelters emit large quantities of pollutants from their stacks. Coal is a common fuel and is the source of some of the pollutants and others (for example, compounds of sulfur, arsenic and tellurium) come from the ore. Major efforts have been made, at great expense, to control smelter pollutants but as a practical matter, they can not be eliminated entirely. In some countries, such as the U.S. and Canada, approval from environmental authorities to continue operation of smelters or to build new smelters, became a serious concern to the industry.
The trend away from smelting caused increased interest in improving electrowinning processes. One area of substantial research was a search for new anodes to replace lead alloy anodes. Particular alloys had evolved over the years as optimum for specific uses, but several disadvantages remained. When a potential is applied to lead anodes in sulfuric acid, oxygen is evolved and lead dioxide forms on the surface of the anode. This lead dioxide coating determines the operating voltage of the anode and this voltage is high compared to platinum sheet or platinum black, widely used anodes in research applications. A surface coating of platinum, or some other electrocatalyst having low oxygen over-potential, on lead anodes would result in power saving and this would justify use of an expensive electrocatalyst provided long service life was obtained.
The lead dioxide coating which forms on lead alloy anodes has long service life when kept continuously under a potential. When power is turned off, however, lead dioxide reacts with sulfuric acid to form lead sulfate. When a potential is again applied, powdery lead sulfate spalls off the anode. Some of this powder is carried by movement of the electrolyte to the cathode where it deposits as impurity. Larger particles of lead sulfate drop to the bottom of the cell and must be periodically removed. This process involves labor costs and loss of production time. These factors are a significant disadvantage of lead alloy anodes.
In addition, the mechanism of loss of surface coating from anodes described in the paragraph above mitigates against use of a low overpotential catalyst coating, even an inexpensive one, on lead alloy anodes. After one or two power interruptions, any surface catalytic coating will be removed as lead sulfate spalls off.
These points were well known to researchers in the electrowinning field and most workers decided to eliminate lead anodes completely. The general approach was to use a valve metal substrate and apply an electrocatalyst to the surface of this substrate. Valve metals are metals such as niobium, tungsten, zirconium, titanium and tantalum and alloys of these metals. When energized as anodes, valve metals quickly form an adherent surface layer of insulating oxide. The corrosion resistance of valve metals is due to this surface layer but this layer must be modified to allow the anode to conduct current. Of the various valve metals, titanium is the least expensive and is most widely available in the form of rod, bar, and sheet of various thickness and titanium is the substrate of choice for electrowinning anodes.
Choice of surface electrocatalyst is governed by the requirements that the catalyst must have low overpotential for oxygen evolution and that it must not dissolve (or dissolve at an extremely low rate) in hot, strong sulfuric acid while passing current. These requirements limited researchers to metals of the platinum groups and compounds of these metals.
At the beginning of prior art research on coated titanium anodes for use in electrowinning, there was substantial information available in the literature on such anodes for use in producing chlorine from brine solutions. Typical of this literature are the teachings of U.S. Pat. Nos. 3,632,498, 3,711,384, 3,491,014, 3,775,284, 3,810,770 and 3,751,296. These references teach that electrocatalysts such as ruthenium oxide or platinum-iridium alloys, when applied by precisely controlled methods to titanium, give anodes having very low overpotential for chlorine evolution and that the anodes exhibit long service life in hot, saturated brine. Utility of these anodes has been well established on large commercial scale and anodes are known to have life of five years or longer when operated at current densities in the range of 200 to 300 Amps per square foot.
Commercially available chlorine anodes which use ruthenium oxide have thin coatings which contain RuO.sub.2 ;TiO.sub.2 or RuO.sub.2 :Ta.sub.2 O.sub.5 in proportions of approximately 50:50 by weight. Details of processing are proprietary but it is understood that the coatings are formed by application of a solution of precursor compounds of the coating oxide, followed by heating in air to form the oxides in adherent form. Commercial chlorine anodes coated with platinum-iridium are stated to contain these metals in proportion of 70:30 by weight and processing is understood to also involve solution coating and heating.
Over the past decade, ruthenium has sold at only 10 to 20% of the price of platinum or iridium. For this reason some researchers on electrowinning anodes obtained commercially prepared ruthenium oxide on titanium anodes and evaluated them in sulfuric acid, under accelerated conditions. All found that the anodes passivated rapidly and that the mechanism involved rapid dissolution of ruthenium oxide. Typical of this research are the efforts of Loutfy and Ho as described in U.S. Pat. No. 4,107,025. They undertook to overcome the rapid passivation of a RuO.sub.2 :Ta.sub.2 O.sub.5 coating by overcoating it with a relatively thick coating of an insoluble metal tungstate combined with Ta.sub.2 O.sub.5 and a small amount of RuO.sub.2 or IrO.sub.2. This approach resulted in anodes having long service life, determined in accelerated laboratory tests. However, a large number of coats are necessary and each coat must be fired, so anodes made in this way have substantial labor costs. In addition, anodes which use IrO.sub.2 as part or all of the precious metal of these composite coatings have substantial precious metal cost.
Other researchers on electrowinning anodes used a different approach to achieve long service life titanium anodes made with ruthenium oxide as the principal electrocatalyst. Scarpellino, McEwen and Borner describe in U.S. Pat. No. 4,157,943 a composite electrode in which titanium is first coated with a metal of the platinum group, then coated with a co-deposited layer of ruthenium and iridium, and finally coated with a layer of RuO.sub.2 :TiO.sub.2. Electrodes made in this manner are labor intensive as the first and second layers are applied by electroplating, the anode is then heat treated, and the top coat of RuO.sub.2 :TiO.sub.2 is applied in multiple coats from solution and each coat is heat treated. Also, these anodes use substantial amounts of precious metals. The first layer is preferably palladium and the second layer contains iridium in addition to its major component, ruthenium.
Commercially produced anode coatings of Pt:Ir were also evaluated for performance in sulfuric acid, by some researchers. It was found that Pt:Ir coatings wear at a much lower rate than conventional RuO.sub.2 :TiO.sub.2 chlorine anode coatings but that the wear of Pt:Ir coatings is still too high to allow their use in electrowinning, on economic grounds. An evaluation of this type was performed in the course of the research which led to the present invention.
To summarize the prior art in the field of electrowinning anodes, it has been established that coated titanium anodes which have very long service life when evolving chlorine, have short life when evolving oxygen. A substantial research effort has resulted in anodes having acceptable service life in electrowinning. This has been achieved by use of relatively thicker coatings, containing other precious metals in addition to ruthenium and in some cases by use of processing steps different from those used to produce chlorine anodes. Because of multiple processing steps, very substantial labor is required to produce electrowinning anodes.